Automatic dishwashers normally are either designed and intended for use in homes (referred to as domestic machines) or are designed and intended to be used in restaurants and similar establishments (referred to as commercial machines). Commercial machines normally use high pressure washing fluid (water with a detergent for cleaning and water, with or without a rinse agent, for rinsing), often in the range of 10 psi to 20 psi. Because of the delicacy of many items washed in domestic machines, they normally use low pressure washing fluid, often in the range of 2 psi to 8 psi.
A typical present day domestic dish washing machine has an elongated main spray arm positioned in the lower part of the wash chamber and rotated by pressure of the washing fluid to spray the fluid on the items to be washed. Generally a number of ports or orifices positioned across its upper wall spray the fluid generally upward over the items to be washed. Normally these ports are angled from the vertical in a symmetrical pattern so that the reaction force of the fluid jets spraying from them produce very little, if any, rotary force on the arm. The arm also has one or two small reaction or drive ports, toward the outer ends of the arm, angled so that fluid exiting from them will cause the arm to rotate. Such reaction ports also may spray fluid over the filter to clean it, as is well known in the art.
The reaction torque or force exerted on the spray arm by the fluid flowing from the reaction ports causes the arm to accelerate until an equilibrium speed is reached. Assuming a single drive jet, the drive or reaction torque can be expressed, in simplified terms, as a constant times the diameter of the jet squared times the fluid pressure. It will be understood that only a portion of the torque, depending on the angle of the jet, causes the arm to rotate.
The drive torque is opposed, and an equilibrium speed is obtained, by two opposing torques or forces. First the mounting of the arm has a resistance or friction torque. In order to control such friction and provide smooth, long life operation the mounting normally is through a bearing mechanism. The bearing friction torque can be expressed, in simplified terms, as the net force on the bearing times the coefficient of friction of the bearing times the radius through which the force acts. Second, as the arm rotates the fluid in the each part of the arm must travel a greater linear distance during a unit of time than water closer to the axis of rotation. The force required to provide this acceleration of the fluid, referred to as the coriolis force, tends to slow down the rotation of the arm. The coriolis force or torque can be expressed, in simplified terms, as the mass flow times the radius from the axis of rotation to the exit opening for the fluid times the angular velocity of the arm.
It will be noted that only the coriolis torque is dependent upon the speed of the arm. Thus, as the arm accelerates, the coriolis torque increases until the bearing friction torque plus the coriolis torque equals the drive torque. The system is then balanced and the arm maintains an essentially steady state speed. The design speed easily can be varied by varying the fluid pressure as well as the size of the drive port and its angle relative to a tangent of the circumference of the arm's motion. Domestic dish washing machines are very competitive and it is not practical to provide high cost, close tolerance bearing mechanisms for the spray arms. Thus the friction torque of a particular design dish washing machine may vary significantly from one individual machine to another. In addition, small food particles and other pieces of debris are commonly entrained in the washing fluid and can enter the bearing mechanism, causing the friction to change significantly for at least some period of time. Such variations in the bearing friction torque will result in the spray arm rotating at a different equilibrium speed, either faster or slower than the nominal design speed. To compensate for such variations it is desirable to provide a nominal or design coriolis torque that is at least twice as large as the nominal or design bearing friction torque.
In machines which spray fluid from a long main spray arm, the effects of the changes in the bearing friction torque can be compensated for easily. First, the design of the drive ports or orifices can assure that the drive torque is significantly larger than the bearing friction torque. Second, since the arm is rather long, often in the vicinity of twenty-two inches and the amount of fluid flowing is large, the coriolis torque also is very large compared to the friction torque. Thus variations in the speed of the arm resulting from variations in the bearing friction torque can easily be maintained in an acceptable range.
Such long spray arms have a number of problems. For example washing dishes with such an arm uses a very large volume of fluid in order to keep the pump primed. Effective washing could be obtained with significantly less fluid by using a small spray device which is rotatably mounted adjacent the end of a long rotating main arm in which fluid is sprayed only from the small device for washing purposes, it being understood that the main arm would be rotated by a drive jet. In the past such an approach has been attempted but with limited success, particularly in domestic machines. One reason is that the spray device is much shorter, perhaps on the order of seven inches. This limits the coriolis torque, particularly in machines with relatively low fluid mass flow, as is typical in domestic machines. Therefore, the bearing friction torque was large compared to the coriolis torque and was the determinative torque in balancing the drive torque. Two substandard operation modes were clearly possible in such machines. First, if the bearing torque of a particular machine became large compared to its nominal design value, as if a hard piece of debris entered the bearing mechanism, the spray device would stop rotating and the washing process would be ineffective. Second, if the bearing friction were somewhat less than the nominal design value, the spray device would reach a very high speed before the coriolis torque rose enough that the drive torque was offset. At such high speeds the fluid leaving the spray device was likely to break up into an ineffective mist.